[BUG] fatal: transport 'file' not allowed during submodule add

[<rsbecker@nexbridge.com>]

As of 2.39.0, I am now getting fatal: transport 'file' not allowed when
performing a submodule add after a clone -l. The simple reproduce of this
is:

1. Start with an empty bare repository, src.git.
2. Create an empty non-bare repository and set the upstream remote to the
bare repo.
3. Populate the non-bare repository with:
	touch .gitignore &&
	git add .gitignore &&
	touch file1 &&
	git add file1 &&
	git commit -m initial &&
	git remote add origin ../src.git &&
	git push --set-upstream origin master
4. Create another empty bare repository to be used as the submodule,
subsrc.git.
5. Create another empty non-bare repository and set the upstream remote to
the bare repo for the submodule.
6. Populate the non-bare submodule repository with
	touch .gitignore &&
	git add .gitignore &&
	git commit -m initial &&
	git add .gitignore &&
	touch file2 &&
	git add file2 &&
	git commit -m initial &&
	git remote add origin ../subsrc.git &&
	git push --set-upstream origin master
7. Clone the main repo using -l or without it (makes no difference):
	git clone -l src.git dest
8. Attempt to add the submodule:
	cd dest &&
	git submodule add -- ../subsrc.git subsrc

This results in:
Cloning into 'dest'...
done.
Cloning into '/home/randall/dest/subsrc'...
fatal: transport 'file' not allowed
fatal: clone of '/home/randall/subsrc.git' into submodule path
'/home/randall/dest/subsrc' failed

This happens for any submodule add on the same system. Some online research
indicates that there was a security patch to git causing this, but I can't
find it. This does not seem correct to me or how this improves security.
Help please - this is causing some of my workflows to break.

Thanks,
Randall
--
Brief whoami: NonStop&UNIX developer since approximately
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-- In real life, I talk too much.